My wife bought my kid a gizmo gadget phone watch. She always knows where the kid is at, plugged in mommy contact and grandparent contact. The kid can call any time. Has instant messaging and voice messaging and is also for if daddy needs help. ( I take it at swim class, Girl Scouts and times she don't need it)

We looked into it but with Justin's issues I couldn't guarantee that it - or anything it was attached to - would remain on his person.

If this is in regard to your child's recent elopement while I can empathize I would be proactive and consult with your local police department if your child has a formal diagnosis for autism spectrum disorder and shows a recurring obsession for elopement. Have you looked into the AWAARE collaborative initiative and the associated Big Red Safety Toolkit?